EPSC Abstracts Vol. 12, EPSC2018-284, 2018 European Planetary Science Congress 2018 EEuropeaPn PlanetarSy Science CCongress c Author(s) 2018

Space weathering in enstatite single crystals: Femtosecond laser pulse experiments simulate micrometeoroid impacts

Doreen Schmidt (1), Kilian Pollok (1), Gabor Matthäus (2), Harald Mutschke (3), Stefan Nolte (2) and Falko Langenhorst (1) (1) Institute of Geosciences, Friedrich Schiller University (FSU), Carl-Zeiss-Promenade 10, 07745 Jena, Germany (2) Institute of Applied Physics (IAP), FSU, Albert-Einstein-Str. 15, 07745 Jena, Germany (3) Astrophysical Institute and University Observatory (AIU), FSU, Schillergässchen 2-3, Jena 07745, Germany ([email protected])

Abstract sliced plane-parallel to a thickness between 0.5 and 1.0 mm and double sided polished. Oriented slices of enstatite single crystals were irradiated by femtosecond laser pulses. Reflectance Laser irradiation was performed on the polished -3 spectra of the irradiated surfaces show distinct surfaces under vacuum (10 mbar) using a darkening compared to the unprocessed surfaces as a Ti:sapphire laser at 800 nm wavelength. Each pulse 2 typical feature of space weathered material. The had a spot size of 38 µm (1/e ) and a duration of microcraters contain a glass layer at the surface, but 100 fs. We used a laser energy of 3 mJ resulting in a 15 2 there is no formation of iron nanoparticles. The crater laser intensity of 10 W/cm . This reproduces well surfaces show different fracturing depending on their the spatial and temporal conditions of natural orientation. Also, the microstructures in the depth of micrometeoroid impacts. 20 x 20 shots were the craters and therefore the deformation mechanisms executed with a spacing of 100 µm to produce a grid 2 differ according to the orientation. with an irradiated area of 2 mm .

NUV-VIS-NIR spectral measurements were 1. Introduction performed in reflectance at near-normal incidence Space weathering, occurring on airless bodies in the with a Perkin Elmer Lambda 19 spectrometer. The solar system, alters the surfaces of planetary spectral range of 200-2,500 nm was measured with a scan speed of 120 nm/min and a 1 nm step size. The materials by micrometeoroid bombardment and solar acquired spectra were corrected with respect to a wind irradiation. The effects of space weathering are calibrated aluminum mirror. responsible for the differences in reflectance spectra between pristine and weathered planetary materials The recovered material was investigated with SEM such as darkening and reddening [1]. Pulsed laser (FEI Quanta3D FEG) and TEM (FEI Tecnai G 2 experiments are known to reproduce space FEG). Sample preparation for TEM was done with weathering effects on olivine like melting, the focused ion beam (FIB) technique by cutting 100 nm formation of iron nanoparticles and deformation thin lamellae perpendicular to the irradiated surface. features like dislocations [2] as well as other planar shock effects [3]. Comparable observations are made on natural olivines in lunar or asteroidal samples 3. Results [4,5]. In this context we pursued further laser 3.1 Surface Analysis of the Microcraters experiments on pyroxene, which is besides olivine one of the most important planetary minerals. The laser single shots produced mainly spherical to partly irregularly shaped microcraters with a distinct 2. Material and Methods splash-like layer. There are different amounts of radially ejected material. The samples parallel to Kilosa enstatite containing about 4 wt% Fe (En93) (001) and (210) show strong fracturing at the crater was cut perpendicular to the crystallographic axes, i.e. surfaces, subordinately also the (100) cut sample. parallel to the (100), (010) and (001) planes. The craters produced on the (001) plane have cracks Additionally, samples were prepared parallel to the mainly subparallel to the traces of the cleavage (210), (211) and (301) planes. The samples were planes on {210} and (100) with smaller subspherical the Martian Meteorite Allan Hills 84001 [6]. We did cracks at the crater rim. The fractures of the (210) not observe the formation of iron nanoparticles, sample show a preferred orientation parallel to the which are generally regarded to be the main cause for [001] direction, whereas the (100) sample is fractured the spectral darkening and reddening in naturally mainly subparallel to [010] and [001]. There are no space weathered pyroxene [5] or have been observed fractures at the crater surfaces of the samples parallel in other experimentally irradiated pyroxene powder to the (010), (301) and (211). pellets [7]. 3.2 Spectroscopy Acknowledgements Reflectance spectra of the processed samples show a We thank the DFG for support within the Leibniz decrease in intensity with respect to the unprocessed program (LA830/14-1) and the research unit FOR surfaces. This is known as spectral darkening. If the 2285 “Debris Disk in Planetary Systems” reflectance at longer NIR wavelengths is less reduced (LA830/20-1). than in the shorter UV-VIS range, the spectrum gets redder. This effect is observed in most samples, but References the intensity of reddening varies strongly between the differently orientated samples. [1] Bennett, C. J. et al.: Space-Weathering of Solar System Bodies: A Laboratory Perspective, Chemical reviews 3.3 Microstructures 113:9086–9150, 2013.

TEM investigations of the craters reveal a layered [2] Fazio, A. et al.: Femtosecond laser irradiation of olivine structure independent of the sample orientation. From single crystals: Experimental simulation of space weathering, Icarus 299:240–252, 2018. top to bottom there is a glass layer, a dominant mechanically deformed layer containing shock [3] Seydoux-Guillaume, A.-M. et al.: Dominance of defects and the undeformed substrate. All samples mechanical over thermally induced damage during show parallel planar lamellae, that are amorphous in femtosecond laser ablation of monazite, European Journal the upper part of the deformed layer and become of Mineralogy 22:235-244, 2010. thinner towards the bottom until they turn to microfractures or stacking faults. At the interface to [4] Noble, S. K. et al.: The Microstructure of a the glass layer there are always two conjugated Micrometeorite Impact into Lunar Olivine, Space directions of the amorphous features, which have an Weathering of Airless Bodies: An Integration of Remote angle of 30-50° to the surface normal. Deeper in the Sensing Data, Laboratory Experiments and Sample deformed layer the samples have either domains with Analysis Workshop, Houston, USA, 2-4 November 2015, subparallel lamellae in different orientations, or the LPI Contribution No. 1878, Abstract 2034. layer contains homogeneously lamellae with one [5] Noguchi T. et al.: Space weathered rims found on the dominant or two conjugated directions. Activated surfaces of the Itokawa dust particles, Meteoritics & cleavage planes can only be observed in the (210) Planetary Science 49:188-214, 2014. sample, where the lamellae are parallel to the (100) cleavage. The transformation to clinoenstatite can be [6] Barber, D.J. and Scott, E.R.D.: Shock and thermal seen in the (301) sample. It occurs in the lower part history of Martian meteorite Allan Hills 84001 from of the deformed layer associated with microfractures transmission electron microscopy, Meteoritics & Planetary and dislocations. Science 41: 643-662, 2006.

[7] Nakamura, K. et al.: Laboratory Simulation of Space 4. Discussion Weathering: Comparison Study of Microstructures of the Laser Irradiated Olivine and Pyroxene, 65th Annual The main deformation features in our shocked Meeting of the Meteoritical Society, Los Angeles, USA, enstatite are microfractures and lamellae that 21-26 July 2002, Abstract 5186. probably represent shear planes. Frictional heating along these shear planes leads to amorphization. These experimentally produced structures are in good agreement with observations in natural samples like